Remove the gateway module

[JorikSchellekens]

Access to the gateway will soon be shut down on the main instance of Starknet. We do not expect any standardisation or space for the Gateway long term. However, appchains may still be interested in accessing their data via the gateway.

We should discourage the use of the gateway in favour of the full RPC, even in the case of Appchains. I recommend removing the gateway module, encouraging entities with access to a sequencer to run a full node.